Thousands of Russians have rallied in Moscow and St. Petersburg in the last two days, facing off against tens of thousands of police and Interior Ministry troops. Hundreds of protesters have been detained in both cities.

Gorbachev told the Interfax news agency that authorities must hold a fresh election or deal with a rising tide of discontent.

Prime Minister Vladimir Putin`s United Russia party won less than 50 percent of Sunday`s vote, a steep fall from its earlier two-thirds majority, according to preliminary results. But opposition parties and international observers said the vote was marred by vote-rigging, including alleged ballot-box stuffing and false voter rolls.

"More and more people are starting to believe that the election results are not fair," he told Interfax. "I believe that ignoring public opinion discredits the authorities and destabilizes the situation."

Gorbachev added that authorities "must admit that there have been numerous falsifications and ballot stuffing."

Sunday`s parliamentary vote suggested Russians are tiring of Putin and his United Russia party, which has dominated all other political forces in Russia for the past dozen years and earned a reputation for corruption.

Putin, meanwhile, officially registered Wednesday to run for the presidency in March, but the unusually sustained protests of the past two days suggested his drive to retake the job he held from 2000-2008 may not go as smoothly as he had expected.

More opposition rallies were expected Wednesday, along with another new pro-Putin gathering in central Moscow.

Thousands of security forces were out in the Russian capital and helicopters roamed the sky Wednesday, a show of force following two days of protest.

Authorities said Tuesday at least 51,500 police officers and 2,000 Interior Ministry troops have been deployed in Moscow since the election. Unlike the police, Interior Ministry troops are an armed force, largely manned by conscripts.

The Russian Union of Journalists condemned police violence and called for a probe into the dozens of attacks and arrests of journalists, describing them as "an attempt to gag and intimidate society."

On Wednesday, two video journalists working for the state-owned RIA Novosti news agency were briefly detained outside the Election Commission building where Putin was handing in his application to run.

The allegations of election fraud have fired up the opposition, which has long seen its protests crushed and its pleas ignored by the Kremlin-dominated media.

On Facebook, more than 12,000 people signed up to a page announcing an opposition rally for Saturday — and many of them have never taken part in political demonstrations

Mariya Boyarintseva, a 24-year-old event manager, said that she has never been to a political rally before but she was going to Saturday`s protest.

Boyarintseva said she didn`t go to rallies Monday or Tuesday in Moscow — which ended with clashes with police and hundreds detained — because "it felt a bit scary."